Python tutorial 39 custom exceptions in python programming

### python tutorial 39: custom exceptions in python programming

in python, exceptions are a way to handle errors that occur during the execution of a program. while python comes with several built-in exceptions, there may be cases where you want to define your own exceptions to handle specific error scenarios in your application. this is where custom exceptions come into play.

#### what is a custom exception?

a custom exception is a user-defined exception that extends the base exception class. by creating your own exception classes, you can provide more meaningful error messages and handle errors in a more controlled manner.

#### creating a custom exception

to create a custom exception in python, you need to define a new class that inherits from the built-in `exception` class or one of its subclasses. here’s how to do it:

1. **define the exception class**: create a new class that inherits from `exception`.
2. **add an `__init__` method**: you can customize the initialization method to accept additional arguments, such as error messages.
3. **override `__str__` method (optional)**: you can override the `__str__` method to provide a custom string representation of the exception.

#### example: custom exception implementation

let’s implement a simple example of a custom exception. in this example, we will create a `negativenumbererror` exception that will be raised when a negative number is encountered in a mathematical operation.

### explanation of the code

1. **custom exception class**:
- `negativenumbererror` extends the built-in `exception` class.
- it takes a `value` parameter during initialization and overrides the `__str__` method to provide a custom error message.

2. **function to calculate square root**:
- the `calculate_square_root` function checks if the input number is negative. if it is, it raises the `negativenumbererror`.

3. **main function**:
- in the `main` function, we have a list of numbers, some of which are negative.
- ...

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